The Fragile X Mental Retardation Protein (FMRP) is hypothesized to be involved in the regulation of mRNA transport and/or translation at synapses. Synaptic dysfunction in fragile x syndrome is believed to result from dysregulation of mRNA in dendrites. However, despite the previous identification of many mRNAs that are bound by FMRP in vitro, it was unknown whether any of the interactions between FMRP and these mRNAs occur in dendrites and at synapses. In the previous funding period, we proposed to test the hypothesis that FMRP is required for the glutamatergic regulation of mRNA transport in dendrites and its translation at synapses. Importantly, we demonstrated distinct roles for FMRP in both the regulation of mRNA transport in dendrites and translation at synapses. An inherent difficulty in studying FMRP function in dendrites was the lack of suitable high-resolution microscopic technology to visualize and quantify mRNA transport and local translation. We have made substantial progress in the refinement of FISH methodology, which led to the discovery that several FMRP associated mRNAs are localized to dendrites in vitro and in vivo. The phosphorylation status of FMRP was shown to provide a means to regulate translation downstream of gp1- mGluR activation. This renewal application will test the hypothesis that FMRP trafficking, posttranslational modification and association with specific miRNAs are involved in the mechanism and regulation of mRNA transport and local protein synthesis. Aim 1 will investigate whether phosphorylation and/or degradation of FMRP is regulated by gp1 mGluRs to influence FMRP dynamics and mRNA trafficking. Aim 2 will characterize the molecular mechanism of FMRP-mediated translational regulation in dendrites. We will assess the role of FMRP phosphorylation in miRNA-mediated translational regulation. Lastly, we will further investigate the hypothesis that FMRP directly regulates the local synthesis of proteins involved in neuronal signaling pathways downstream of gp1-mGluR activation. Aim 3 will focus on the role of FMRP in PI3K mediated signal transduction and regulation of protein synthesis. This research will provide new insight into mechanisms of protein synthesis-dependent synaptic plasticity underlying learning and memory, and may lead to the development of new therapeutic strategies for fragile x and other forms of mental illness.